In order to produce lower cost solar cells and hence expand large-scale electrical applications of solar electricity, it is important to have lower cost substrate materials for making the solar cell. One preferred method for achieving this in the case of crystalline silicon is through the growth of crystalline silicon ribbon in a continuous process. See, e.g., U.S. Pat. Nos. 4,594,229; 4,627,887; 4,661,200; 4,689,109; 6,090,199; 6,200,383; and 6,217,649.
In these patents, continuous silicon ribbon growth is carried out by introducing two high temperature material strings up through a crucible that includes a shallow layer of molten silicon. The strings serve to stabilize the edges of the growing ribbon and the molten silicon freezes into a solid ribbon just above the molten layer. The molten layer that forms between the strings and the growing ribbon is defined by the meniscus of the molten silicon.
U.S. Pat. Nos. 6,090,199 and 6,217,649 describe a method and apparatus for continuous replenishment of the feedstock material in a continuous silicon ribbon. In order to make a useful electrical device, it is necessary, in the case for a solar cell to dope the feedstock material so as to eventually form a p-n junction. In a typical method of doping silicon, a small material so as to eventually form a p-n junction. In a typical method of doping silicon, a small quantity of either the dopant itself or a concentrated form of the dopant in silicon is added to a silicon melt. The addition of the dopant in this manner in batch processes is not readily compatible with continuous ribbon growth techniques such as that discussed above. In connection with these patents, it is necessary to have silicon feedstock material that is composed of small silicon particles.